Well that would have to depend on several factors.
The human body has approx 70ml of blood for every kg of patient.
150 lb.....68kg......4,760ml.
A previously healthy individual should start to show signs of hypovolemic shock after loosing approx 20% of its total blood volume.
4,760ml........20%........952ml.........a little less than a 1000ml bag of fluid.
hope that helps.